The present invention relates to an electron gun used in a color picture tube, particularly an in-line electron gun, and more particularly to the structure of electrodes forming the main lens of the electron gun.
One of factors which have a great influence on the focussing characteristics of a color picture tube, is the aperture diameter of the main lens of the electron gun. In order to obtain satisfactory focussing characteristics, it is desirable to make the aperture diameter of the main lens as large as possible.
However, in an in-line gun system, three electron guns for green, blue, and red are arranged on one and the same horizontal plane and united in one body. Such an inline electron gun is placed in a neck tube which has a limited inner diameter. Accordingly, the diameter of lenses formed in the electron guns and the interval of the lenses are restricted by the diameter of the neck tube, and thus it is very difficult to make large the aperture diameter of these lenses. The above problems will be explained below in more detail.
The focusing characteristics of a picture tube are greatly affected by the magnification and aberration of the main lens, which depend strongly upon the converging effect of the lens. In the picture tube, once the scanning area and maximum deflection angle of the electron beam are given, the distance from the main lens to the focal surface is determined. If the converging effect of the lens is weakened under the condition that the distance from the main lens to the focal surface is constant, the magnification of the lens will be reduced. Further, if another condition that the spread of the electron beam in the main lens is made less than a certain value, is added to the abovementioned condition to prevent the deflection aberration from increasing, the incident angle of the electron beam at the main lens must be reduced. The diameter .delta. of the disk of the least confusion caused by the spherical aberration which is most dominant in aberrations of the main lens, is given as follows: ##EQU1## where .alpha..sub.i is the incident angle of the electron beam, M the lens magnification, and C.sub.sp the coefficient of spherical aberration. It is known from the above equation that the spherical aberration can be reduced by reducing the incident angle of the electron beam.
As mentioned above, by weakening the converging effect of the main lens, the lens magnification and spherical aberration can be reduced, and thus the focusing characteristics can be improved. One method for weakening the converging effect is to provide larger diameters for the apertures of G3 and G4 electrodes constituting the main lens.
However, the in-line electron gun includes three lenses for red, green, and blue, and these lenses are arranged on a horizontal plane. Accordingly, the diameter of apertures for forming these lenses is required to be less than one-third of the inner diameter of the neck portion of a glass envelope accomodating the electron gun. The allowable value of the above aperture diameter will further be reduced when the thickness of the electrode and the machining accuracy thereof are considered. If the inner diameter of the neck portion is increased to make large the above-mentioned allowable value, the deflection power will increase. In general, the enlargement of aperture diameter increases the distance between the axes of the apertures, therefore, the distance S between electron beams, and deteriorates the converging characteristics. Taking these matters into account, the aperture diameter has been made as large as possible in a limited range, and it is very difficult to further increase the aperture diameter.